FIELD OF THE INVENTION
The invention relates to a method of producing an aluminum film on a substrate.
Aluminum has been the metal used nearly exclusively for conductor tracks in semiconductor technology, because it is very well able to meet the demands made of such conductor tracks.
However, aluminum also has several negative properties, the effects of which have not yet been completely eliminated.
One of these particularly negative properties is the pronounced electromigration in aluminum conductor tracks. The term electromigration is understood to mean the migration of material in conductor tracks under the influence of an electric current. Because of the impacts of moving electrons against the positively charged aluminum core of the crystal lattice, a depletion of material in the direction of the electron flux takes place. This depletion of material can cause a complete local interruption of a conductor track and hence the total failure of an integrated circuit.
A further phenomenon, namely so-called stress migration, can also occur in aluminum conductor tracks. This is understood to mean the migration of aluminum atoms under the influence of internal mechanical stresses at elevated temperature. Stress migration, too, can ultimately lead to an interruption of a conductor track and thus to the total failure of an integrated circuit.
The art has found that the addition of approximately 0.5 to 5% by weight of copper to the aluminum is a viable defense against stress migration and electromigration.
With very narrow aluminum conductor tracks (width .ltoreq.1 .mu.m) in very large scale integrated circuits, however, cracks and interruptions in the conductor tracks are found despite the aforementioned alloy addition of copper. The cracks and line breaks occur ever more often, even without any current load, and after passive temperature storage at low temperatures (125.degree. C.).
Multilayer metallizings with SiO.sub.2 dielectrics as underlays and SiO.sub.2 /SiN.sub.x cover layers in the upper metal levels are especially affected. The reason for this are the very strong extrinsic tensile stresses that occur there. They have been found to be caused by the later gaplike cracking of the aluminum conductor tracks in the cooling phase of the annealing process or in the polyimide cross-linking. The process-dictated cooling characteristic there has the effect that during the cooling, flat copper islets are formed--so-called Guinier-Preston phases--in the grown aluminum grains. In the previously conventional cooling phases, the copper atoms migrate out of the interior of the aluminum grains to the grain boundaries, where they convert into the stable Al.sub.2 Cu-.theta.-phases. They leave behind open lattice planes or lattice planes that are greatly disturbed by offsets. These disturbed planes cannot be entirely cured by the aluminum atoms.